In general, a motor vehicle transmission includes an input shaft and an output shaft. The input shaft is typically coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gear set. The transmission employs a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controllable to vary the speed ratio between the transmission input and output shafts.
Transmissions are typically characterized by a plurality of fixed speed ratios. Each of the fixed speed ratios is achievable by engaging a particular combination of clutches. An electrically variable transmission includes at least one motor/generator, and is typically characterized by at least one electrically variable mode or range of operation in which the speed ratio between the input shaft and the output shaft is not fixed, but instead varies with the speed of the rotor of the motor/generator. An electrically variable transmission may be configured such that multiple electrically variable modes or ranges are achievable by engaging particular combinations of clutches. Other combinations of clutches in an electrically variable transmission may result in fixed speed ratio modes.
Shifting from a currently established fixed ratio or electrically variable mode to a new fixed ratio or electrically variable mode involves, in most cases, disengaging a clutch (off-going clutch) and engaging another clutch (on-coming clutch). Clutches may be engaged by the action of pressurized fluid against a piston in a clutch apply chamber. Accordingly, transmissions typically include a hydraulic circuit for supplying pressurized fluid to the apply chambers of the clutches.